Microorganisms are involved in the manufacture of food and feed products including most dairy products. Bacterial cultures, in particular cultures of bacteria that are generally classified as lactic acid bacteria, are essential in the making of all fermented milk products, cheese and butter. Cultures of such bacteria may be referred to as starter cultures and they impart specific features to various dairy products by performing a number of functions.
Dairy starter cultures are generally composed of lactic acid bacteria. In the present context, the expression “lactic acid bacteria” (LAB) designates a group of Gram positive, catalase negative, non-motile, non-sporulating, microaerophilic or anaerobic bacteria which ferment sugars with the production of organic acids, including lactic acid as the predominantly produced acid, formic acid and propionic acid. In the present context lactic acid bacteria comprise of a number of bacterial genera within the phylum Firmicutes. The genera Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Lactosphaera, Leuconostoc, Melissococcus, Oenococcus, Pediococcus, Streptococcus, Tetragenococcus, Vagococcus and Weissella are recognized as LAB. Also lactic acid-producing Gram-positive bacteria belonging to the phylum Actinobacteria such as the genera Aerococcus, Microbacterium and Propionibacterium as well as Bifidobacterium are in the present context considered as LAB. The industrially most useful lactic acid bacteria are found among Lactococcus species, Streptococcus species, Enterococcus species, Lactobacillus species, Leuconostoc species, Bifidobacterium species and Pediococcus species.
In addition to the their use in the dairy industry lactic acid bacteria cultures also find widely use in the meat processing industry as well as a number of other industries.
Commercial starter cultures may be distributed as frozen cultures. Highly concentrated frozen cultures are commercially very interesting since such cultures can be inoculated directly into the fermentation medium (e.g. milk or meat) without intermediate transfer. In others words, such highly concentrated frozen cultures comprise bacteria in an amount that makes in-house bulk starter cultures at the end-users superfluous. A “bulk starter” is defined herein as a starter culture propagated at the food processing plant for inoculation into the fermentation medium. Highly concentrated cultures may be referred to as direct vat set (DVS)-cultures. In order to comprise sufficient bacteria to be used as a DVS-culture at the end-users, a concentrated frozen culture generally has to have a weight of at least 50 g and a content of viable bacteria of at least 109 colony forming units (CFU) per g.
An important issue in the practical use of frozen cultures is the convenience of the actual handling of the cultures. Whereas cultures frozen “on block” are difficult to handle it has been found that cultures frozen in pellets are very easy to handle both for the producer and the consumer.
Consequently, a thriving market for highly concentrated pellet frozen cultures—so-called frozen direct vat set (F-DVS)-cultures—has formed.
A number of publications concerned with the viability of frozen cultures have occurred.
Chavarri et al. (1988) describes that the viability of a frozen pure Streptococcus lactis culture can be improved by addition of 5% lactose or 5% sucrose.
Cárcoba et al. (2000) describes that the viability of a frozen pure Lactococcus lactis subsp. lactis culture can be improved by addition of different cryoprotective agents such as sugars (lactose, sucrose and trehalose), glutamic acid and gelatin.
U.S. Pat. No. 4,140,800 (Kline), describes that the viability of freeze-dried cultures can be improved by addition of different cryoprotective agents. Also the viability of frozen cultures added lactose, sucrose or maltose are discussed.
WO00/39281 (Kringelum et. al.) describes that the viability of a non-frozen, liquid starter culture can be improved by addition of different cryoprotective agents, and